It is desirable when moving a load by means of a motor to employ a device to hold the load in the event the motor becomes disconnected from the load. For example, a load may be supported on the end of a cable and the cable wound around a drum which is driven by a motor through a drive shaft. During the raising of the load, for example, it may occur that the motor becomes accidentally disconnected from the drive shaft. If there are no means for braking the drive shaft or drum, the load can uncontrollably free fall to the ground.
Another example involves the leading edge slats of an aircraft. The slats are extended and retracted by one or more rotary actuators. The rotary actuators are driven by a hydraulic motor which is connected to the rotary actuators by a torque tube. When the slats are extended in the presence of reverse acting air loads, it is important that the slats remain extended in the event of failure of the hydraulic motor or structural failure of the torque tube. Furthermore, when the slats are retracted and subjected to overrunning air loads, it is important that the slats retract in a controlled manner in order to prevent damage to the slat assembly and other related components. Conventionally, there exists conventional brakes which sense when normal operation of the slats has been interrupted, and thereby lock the slats in place.
Other conventional brakes have been disclosed in U.S. Pat. No. 3,285,377 by Rasmussen in which a brake assembly includes rollers which are displaced by a reverse torque to initiate braking.
U.S. Pat. No. 3,367,456 by Behnhoff discloses a brake for limiting the torque transmitted in a drive train by using a ball-ramp drive connection to initiate braking.
Furthermore, U.S. Pat. No. Re. 26,540 and U.S. Pat. No. Re. 27,327 both by Kalns disclose a bidirectional no-back drive device which operates to spread apart a pair of shoes in the presence of reverse torque in order to initiate braking.
And, U.S. Pat. No. 4,597,477 by Miller discloses a bidirectional brake having a ball-ramp assembly which is used to operate a brake to prevent an output shaft from driving an input shaft.
A limitation with regard to conventional bidirectional brakes is apparent during the presence of a so-called "overrunning load". That is, in the presence of an overrunning load, a rotational force is present at the output shaft which acts in the same direction that the output shaft is being rotated by the input shaft. This force wants to rotate the output shaft at a faster rate than it is being driven by the input shaft. Conventional bidirectional brakes operate to brake the output shaft so that it does not overrun the input shaft.
However, in the presence of an overrunning load, conventional bidirectional brakes tend to brake the output shaft in a somewhat jerky manner. This is because the "breakout friction" of the conventional brake is much higher than its "running friction". More specifically, the friction applied by the conventional brake to hold the output shaft is much higher than the shaft friction present when the shaft is rotating. Thus, when the conventional brake releases the output shaft and the torque of the output shaft overcomes the braking friction, the output shaft moves rapidly from its braked state to its rotational state, generating large, momentary rotational forces which are transmitted to elements downstream of the output shaft.